Re: [virtio-dev] Re: [PATCH v2 4/5] virtio-mmio: add MSI interrupt feature support

["Liu, Jing2" <jing2.liu@linux.intel.com>]

On 2/11/2020 11:17 AM, Jason Wang wrote:
>
> On 2020/2/10 下午5:05, Zha Bin wrote:
>> From: Liu Jiang<gerry@linux.alibaba.com>
>>
>> Userspace VMMs (e.g. Qemu microvm, Firecracker) take advantage of using
>> virtio over mmio devices as a lightweight machine model for modern
>> cloud. The standard virtio over MMIO transport layer only supports one
>> legacy interrupt, which is much heavier than virtio over PCI transport
>> layer using MSI. Legacy interrupt has long work path and causes specific
>> VMExits in following cases, which would considerably slow down the
>> performance:
>>
>> 1) read interrupt status register
>> 2) update interrupt status register
>> 3) write IOAPIC EOI register
>>
>> We proposed to add MSI support for virtio over MMIO via new feature
>> bit VIRTIO_F_MMIO_MSI[1] which increases the interrupt performance.
>>
>> With the VIRTIO_F_MMIO_MSI feature bit supported, the virtio-mmio MSI
>> uses msi_sharing[1] to indicate the event and vector mapping.
>> Bit 1 is 0: device uses non-sharing and fixed vector per event mapping.
>> Bit 1 is 1: device uses sharing mode and dynamic mapping.
>
>
> I believe dynamic mapping should cover the case of fixed vector?
>
Actually this bit *aims* for msi sharing or msi non-sharing.

It means, when msi sharing bit is 1, device doesn't want vector per queue

(it wants msi vector sharing as name) and doesn't want a high interrupt 
rate.

So driver turns to !per_vq_vectors and has to do dynamical mapping.

So they are opposite not superset.

Thanks!

Jing


> Thanks
>
>
>
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